It is known that encroachment of oxygen into an electrochemical cell causes corrosion of the zinc cathode, and corresponding loss of performance of the cell. It is also known that loss of moisture out of the cell, and consumption of electrolyte by reaction with encroaching oxygen, causes loss of potential electrical performance of the cell.
Generally, the cell of interest includes a metal can as an anode, a cathode comprising manganese dioxide, an electrolyte, a separator, and a current collector. A retaining washer is preferably positioned at the top of, and covering, the cathode mix. Typically the retaining washer is spaced from outer elements of the cell, including any outer seal, by an intervening inner chamber.
Various attempts have been made, and significant research resources have been expended in attempts to control loss of moisture from especially the top of the cell, and to inhibit ingress of oxygen into the cell. Examples of such attempts are shown in the following United States Patents.
U.S. Pat. No. 4,632,887 Jung teaches using an adhesive sealing substance 8 between his carbon rod 9 and a high elasticity (retaining) washer 10.
U.S. Pat. No. 2,773,926 Glover teaches using either asphalt or microcrystalline wax to coat all or a portion of the inner chamber of the cell.
U.S. Pat. No. 5,079,108 Annen et al teach using a heat shrinkable plastic seal strip between (i) the seal washer 12 and the lip 19 of the cell and (ii) the retaining washer 14.
U.S. Pat. No. 3,179,537 Reilly teaches using a wax coating as a seal on the carbon rod and the inner surfaces of his cap 20, and washers 24 and 26.
U.S. Pat. No. 3,922,178 Winger teaches using a fatty polyamide as a type of seal, to protect some interior surfaces of the inner chamber from being wetted by alkaline electrolyte.
U.S. Pat. No. 2,169,702 Marsal teaches placing a seal washer on the interior of the cell, above the retaining washer, then heating the cell in the area of the seal washer in order to melt the seal washer material, and cause it to flow into the desired respective joints of the cell components in the inner chamber. As taught, the cell is inverted before heating in order to cause the heated seal material to flow, and thus be distributed on the bottom surfaces of cap B, cover A, and washer W, namely at the top of the inner chamber.
U.S. Pat. No. 4,001,044 Miyoshi et al teach an interior seal 25 of wax on the retaining washer, and an outer seal 25a at the joints of the carbon rod 12, the plastic plug 14a, and the plastic jacket 15a. The outer seal can be a variety of materials, including Vaseline.
Despite these advances, dry cells continue to be plagued with an unacceptable degree of moisture loss from of the cell, and ingress of oxygen into the cell. As moisture is lost from the cell, the electrochemical performance of the cell declines. As oxygen enters the cell, it reacts with the electrolyte at the surface of the metal can, causing corrosion of the can, and consumption of the can, thus reducing the effectiveness of the can as an anode.
It is an object of this invention to provide a novel interior seal, in the inner chamber of the cell, which provides a superior barrier to loss of moisture from the cell, and a superior barrier to oxygen ingress into the cell.
It is a further object to provide an interior seal which either covers, or preferably wicks into, the separator above the retaining washer, thus preventing ingress of oxygen through the separator, to the surface of the metal can, and also preventing upward advance of the electrolyte through the separator adjacent the can wall.
It is yet another object to provide an interior seal compound and seal having a first liquid component which covers or wicks into the separator and a second semi-solid component which fixes the position of the compound in the inner chamber on the retaining washer (or cathode mix) and the separator.
It is still another object to provide such an interior seal compound and seal wherein the liquid and semi-solid components have such an affinity for each other that the immobility of the semi-solid component retards the mobility of the liquid component, and thus limits substantially the zone of migration of the liquid component essentially to covering or penetrating the retaining washer and covering or penetrating the separator above the retaining washer or cathode mix.